Reasonable (cheap) cell phone service providers question

I have an iPhone 4S and am out of contract right now. My phone is acting up but I can swap it for $50.....I could upgrade but I'd rather not pay $100 a month on my phone if I don't have to so what are my options? What cheap plans have good coverage?

Please answer me as though you are talking to a retarded 8 year old. TIA.